This is a critical time for HIV prevention programs. Recent recommendations advocate universal HIV antibody testing in order to identify the estimated 25% of HIV-infected individuals who are unaware of their HIV status. In the United States, men who have sex with men (MSM) still represent the group with the greatest risk for HIV acquisition despite a high penetrance of testing. Efforts to prevent HIV transmission must therefore increase the frequency of HIV testing among MSM and thereby decrease the time interval that infected individuals are unaware of their status and their potential for transmission. However, strategies that increase the frequency of HIV testing in a population with high incidence will paradoxically increase the rate of false-negative test results in the window period following HIV acquisition, requiring the use of antibody tests with the highest sensitivity in early infection or concurrent nucleic acid amplification testing (NAAT) to shorten the window period further. Of 14005 specimens from MSM tested by Public Health - Seattle & King County (PHSKC) from September 2003 to June 2008, 328 (2.3%) MSM were HIV antibody-positive, and 36 (0.3%) antibody-negative MSM had acute infection. The OraQuickADVANCE Rapid HIV-1/2 Antibody Test (OraQuick) detected only 153 (80%) of the 192 HIV-infected MSM screened by the rapid test. The proposed studies will increase our understanding of the interactions between the frequency of HIV testing, test sensitivity, and the diagnosis of acute and early HIV infection. In Aim #1, we will evaluate an intervention to increase the frequency of HIV testing by randomizing HIV-negative MSM to either home self-testing with OraQuick or to testing in clinic settings that include NAAT. In Aim #2 we will compare the real-time performance of three rapid HIV antibody tests in order to determine their relative capabilities to detect persons immediately after HIV acquisition. In Aim #3, we will generate data to evaluate the performance of NAAT on specimens (oral fluids and dried blood spots) that could be collected as part of home testing or in other settings without immediate access to NAAT. Cost-effectiveness analyses will be performed to evaluate the costs of different HIV testing strategies per case of HIV infection identified. This application addresses several topics that had been sought to advance prevention opportunities to reduce HIV transmission. These research topics include methods to increase the detection of acute and early HIV infection, including the combination of rapid assays and NAAT, and ways to diagnose acute and early infection in developing countries. The expansion of HIV testing with antibody testing alone may have undesired effects if highly-infectious persons with recent HIV infection receive false-negative test results and are inappropriately reassured. The cross-disciplinary collaboration between PHSKC and the University of Washington will support this translational research to inform local, national, and international public health policy.